GCN Circular 28765

D. A. Kann (HETH/IAA-CSIC), A. de Ugarte Postigo (HETH/IAA-CSIC, 
DARK/NBI), M. Blazek, J. F. Agui Fernandez, C. C. Thoene (all 
HETH/IAA-CSIC), S. Geier, and M. Rivero (both GRANTECAN) report:

We observed the afterglow (Lipunov et al., GCN#28718; Xu et al., 
GCN#28719; Belkin et al., GCN#28725; Breeveld & Lien, GCN#28743; Vinko 
et al., GCN#28750; Kumar et al., GCN#28753) of GRB 201020B (Fermi GBM 
Team, GCN#28702; Fermi GBM detection: Malacaria & Meegan, GCN#28710; 
Fermi LAT detection: Arimoto et al., GCN#28716; AGILE detection: Ursi et 
al., GCN#28714; Konus-Wind detection: Ridnaia et al., GCN#28723) with 
the 10.4m GTC telescope, at Roque de los Muchachos Observatory (La 
Palma, Spain) equipped with OSIRIS. The observation started on 24 
October 2020 at 04:05:16 UT (3.438 days after the GRB trigger, following 
a period of inclement weather) and consisted of 4 x 1200 s with the 
R1000B grism, covering the wavelength range between 3700 and 7800 AA. 
Observations were taken under good conditions.

At the afterglow position, a source is clearly detected. We measure i' ~ 
21.45 mag (AB) vs. a nearby PanSTARRS field star. This represents a 
significant decay compared to the value reported by Vinko et al. 
(GCN#28750). The afterglow is seen to decay with a slope of 1.68, in 
full agreement with Kumar et al. (GCN#28753).

A preliminary reduction of the spectrum shows a clear but featureless 
continuum. There is an emission feature, which we identify as due to the 
[OII]3727 doublet at z = 0.804. Other interpretations (as Hbeta, [OIII] 
or Halpha) would imply other emission lines should be visible, which 
they are not. At the same redshift there could be marginal, weak 
features consistent with FeII, MgII, and MgI. Therefore, we suggest z = 
0.804 as the redshift of the GRB.